Uric acid crystal could inhibit Numb-induced URAT1 lysosome degradation in uric acid nephropathy.

To investigate whether uric acid could regulate urate transporter 1 (URAT1) protein and activity level, we established uric acid nephropathy (UAN) rat model and detected their serum uric acid and URAT1 level with or without the treatment of allopurinol. Results here showed that allopurinol could reduce serum uric acid level in UAN rat model. We further found that in UAN rats, the total and surface URAT1 expression level were both increased while this increase could be blocked by allopurinol treatment. By treating URAT1 stable expressed HEK cell with monosodium urate (MSU) crystals, we found that URAT1 level showed an increase in both total and cell surface level, and it would colocalize more with Rab11 instead of Rab7. Consistently, we also found that the total URAT1 protein level will show an increase in the presence of lysosome inhibitors but not ubiquitin-proteasome inhibitors. Furthermore, we also found that MSU crystal could drive Numb, a clathrin-coated adaptor protein which performs a key function in cell division, out of cell surface and disassociated it from URAT1. Finally, we found that Numb short hairpin RNA (shRNA)-transfected showed a phenocopy as MSU treatment, while Numb-2A mutation over-expression could resist crystal-induced phenotypes. These findings indicated that uric acid crystal could increase URAT1 membrane distribution through inhibiting Numb-induced URAT1 lysosome degradation.